The present invention relates to a lens pattern blank for use in shaping lenses for fitting into eyeglass frames and the like.
Lens pattern blanks are known in the art and are typified by the prior art blank 1 shown in FIGS. 1 and 2. Blanks of this type, although shown in one particular shape, can be square, rectangular or circular. These blanks are generally formed from high impact styrene plastic and have two planar faces 2, 3 and a mounting hub 4 including mounting holes 5-7 which are used to mount the blank on a lens cutting or edging machine.
Conventional blanks include a centering grid pattern thereon including X and Y axes 8, 9 which have as their origin the center C of mounting hole 5. The X and Y axes 8, 9 also include distance indicia 10 thereon to enable the blank to be centered in the lens opening of a frame. This blank is used by the optician or optical lab technician to cut a pattern for a particular frame to be able to cut a round lens to the proper shape to be fit into the frame.
When a pattern is cut manually or mechanically, the most important data that must be supplied to the optician or lab technician is the effective diameter of the pattern which is twice the largest radius from the geometric center and the angle of that radius. This enables the optician to choose a smaller and therefore less expensive lens blank for the prescription.
Conventionally, the length of the longest radius has been obtained by making measurements on the pattern after it has been cut. The grid on the face of the blank is used to center the eyeglass frame over the lens blank when the pattern is being drawn thereon.